1. Field of the Invention
The present invention relates generally to a preliminary treatment for a stainless steel which is to be processed by a cold rolling mill for producing cold rolled stainless steel with high surface brightness or glossiness. More specifically, the invention relates to a method and apparatus for performing preliminary treatment for hot rolled stainless steel strip for obtaining high quality cold rolled stainless steel strip with high surface glossiness through cold rolling process.
2. Description of the Background Art
It is known for producing cold rolled stainless steel strip to perform a preliminary treatment by the process of intermediate annealing and pickling. The preliminarily treated stainless steel strip is cold rolled by means of a Sendzimir mill having a work roll of 100 mm.phi. or less, and so forth. Rolling mill lubricant is supplied to the mill during the cold rolling process. After cold rolling, the stainless steel strip is treated by finishing annealing and pickling. Thereafter, temper rolling is performed with draft of 0.5% to 1.2%.
The cold rolled stainless steel can be generally classified as ferritlic stainless steel, such as SUS430 and austenitic stainless steel, such as SUS304. As is well known, the ferritic stainless steel strip is generally used as a lining material for constructions, surface material of home facilities, or various decorative facilities. For this, such ferritic stainless steel strip must have satisfactorily high surface glossiness or brightness after the temper rolling process. On the other hand, the austenitic stainless steel strip is used for kitchen appliances because of higher corrosion resistance than ferritic stainless steel strip. Such austenitic stainless steel strip is usually subjected to buffing after temper rolling. After the buffing process, the austenitic stainless steel strip must have a satisfactorily high surface glossiness or brightness.
In order to obtain a satisfactorily high surface glossiness or brightness, various cold rolling processes for ferritic and austenitic stainless steel strip have been proposed. For example, Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-227904 and Japanese Patent First Publication (Tokkai) Showa 61-49705 propose improvement of the cold rolling process by seeking optimum work roll diameter and surface roughness of the work roll to be utilized in the cold rolling mill. On the other hand, Japanese Patent First Publication (Tokkai) Showa 58-56013 proposes improvement on intermediate pickling process. Furthermore, Japanese Patent First Publication (Tokkai) Showa 59-107027 proposes addition of grinding after intermediate annealing and pickling process.
Such improvements are generally successful in improving quality of the stainless steel strip and for providing higher surface glossiness or brightness. However, such prior proposals are not satisfactory in providing satisfactorily high surface glossiness or brightness for products, such as the BA product, which is requires very high surface glossiness or brightness.
In the meantime, in recent years, there is a tendency to seek higher production efficiency in producing high surface brightness stainless steel strip. For this requirement, a tandem cold rolling mill having a greater work roll diameter than that of the Sendzimir mill, has tended to be used in place of Sendzimir mill. Such tandem mill with an increased diameter work roll achieves higher efficiency. That is, significant reduction of process time can be achieved by allowing one way rolling utilizing the tandem mill, since the process otherwise required bi-directional rolling in a Sendzimir mill. Furthermore, because a greater amount of rolling mill lubricant can be introduced between the roll surface and the stainless steel strip surface to avoid direct contact of the roll surface onto the strip surface, this successfully prevented the strip from forming defects such as heat-streaks.
On the other hand, a greater amount of rolling mill lubricant introduced between the mating surfaces of the roll and strip tends to cause difficulty in control of surface roughness of the strip. As a result, surface roughness of the stainless steel strip after rolling becomes substantially great. This clearly degrades surface brightness or glossiness of the stainless steel strip.
In order to improve such defect in the tandem cold rolling mill in production of high surface brightness stainless steel strip, there have been proposed various improvements. For example, Japanese Patent First Publication (Tokkai) Showa 61-23720 proposes to perform intermediate annealing and pickling after cold rolling and subsequently to perform cold rolling once again. On the other hand, Japanese Patent First Publication (Tokkai) Showa 61-49701 proposes a cold rolling process in which cold rolling utilizing a small diameter work roll is performed subsequently to cold rolling by a large diameter work roll. Such improvements successfully improve surface quality of the stainless steel strip to be produced through the cold rolling process. However, providing an additional steps, i.e. intermediate annealing and pickling after cold rolling and additional cold rolling by a small diameter work roll, requires additional process time and lower efficiency. Degradation of the production efficiency is significant because intermediate annealing and pickling or cold rolling utilizing a small diameter work roll is performed at substantially reduced line speed in comparison with that in cold rolling utilizing a large diameter work roll.